Quote from peilthetraveler:
The law regarding incest did not come until the time of Moses.
Think of it this way. Just because there was no law saying that its against the law to run stop signs before cars existed, doesnt mean the government "changed its mind" one day when cars were invented and stop signs were needed.
God's law came to men when it was needed. There was no mind changing.
So god was okay with it until he wasn't. That's called a Mitt.